In the practice of interfacial polymerization to produce polycarbonate, a mixture of dihydric phenol and a phenolic chain terminator is phosgenated under interfacial reaction conditions in the presence of an organic solvent. The dihydric phenol is present as an alkali metal salt in an aqueous phase and the phosgene is dissolved in the organic phase. The chain growing reaction is strongly accelerated by a coupling catalyst, such as a tertiary amine. Molecular weight of the polycarbonate polymer is typically controlled by the amount of chain terminator, such as a monophenol, present in the reaction mixture. The chain growing reaction is finished after all of the reactive end-groups, e.g., chloroformate end-groups, have reacted. After the polymerization step is completed, the organic phase comprising the polycarbonate polymer is separated from the aqueous phase and the polycarbonate polymer isolated.
Polymer molecular weight is one of the most important factors that affect polycarbonate polymer properties. As the polymer molecular weight increases, the mechanical properties of the polycarbonate polymer improve. For example, tensile strength, impact resistance, ductility, and other physical properties of the polymer are all improved with increasing polymer molecular weight. However, as the polymer molecular weight increases, the melt viscosity also significantly increases. When the melt viscosity becomes too high, melt processing the polymer becomes difficult or nearly impossible.
Ensuring consistent polycarbonate polymer quality in its manufacturing process is highly desirable. For example, if the polycarbonate polymer molecular weight and/or its molecular weight distribution falls outside a desired and/or targeted range, unacceptable amounts of unusable (sometimes referred to as out of spec or off-grade) polycarbonate may be produced. Minimizing the amount of off-grade polycarbonate provides many benefits to the manufacturer as well as the end user, to name a few, a better product, a more cost effective process, less waste, lower energy consumption and the like.
The molecular weight of a polycarbonate polymer produced in an interfacial process is dependent upon, among other things, the amount of dihydric phenol available for phosgenation and the amount of chain terminator. It has been found that in an interfacial polymerization process for manufacturing polycarbonate polymer the optimal pH is between 8.0 to 10.2, see U.S. Pat. No. 5,380,814. Further, it was determined under such conditions, not all the dihydroxy compound is available for reaction with phosgene. There is some dihydric phenol (which is not present as the bisphenate salt) which is soluble in water. Moreover, varying amounts of the dihydric phenol remain in the solid state, for example, because of inadequate agitation and/or because such solid dihydric phenol may attach itself to the upper portion of the reactor and remain unreacted. The amount that remains solid is neither consistent nor predictable. As a result, the amount of dihydric phenol (and therefore the resulting dihydric phenol to terminator ratio) which is fed to the reactor from its feed tank differs from the amount of dihydric phenol actually delivered through the process stream (resulting in the actual dihydric phenolate to terminator ratio which determines the molecular weight of the polycarbonate) into the polymerization reactor. The fluctuation in the ratio causes undesirable variations in the polycarbonate polymer molecular weight creating off-grade material with unacceptable properties and negative manufacturing cost ramifications.
Therefore, a method of reliably and quickly monitoring and controlling selected parameters which affect polycarbonate molecular weight in the interfacial process so as to ensure consistent product quality and high productivity would be very desirable.